MOLLUSCA — PELECYPODS 213 



food and oxygen. From these sinuses the blood is gathered 

 up by veins and returned to the auricles, the larger part passing 

 through the gills on its return. 



The excretory organs are the nephridia, a pair of dark, U-shaped 

 tubes, of which one arm is glandular and spongy, the kidney, 

 and the other is non-glandular and thin-walled, the bladder. 

 They are situated in the postero-dorsal region. The kidney 

 communicates with the pericardium and the bladder opens into 

 the mantle cavity. Waste material gathered from the body 

 by the blood is extracted from the blood by the kidneys and 

 passed out of the body into the mantle cavity by the exterior 

 opening of the bladder. 



The nervous system consists of three pairs of large ganglia 

 and their connectives : a ganglion on each side of the oesoph- 

 agus, united by a nerve cord and connected by other nerve 

 cords with the two pedal ganglia in the base of the foot, and the 

 two visceral ganglia near the posterior adductor muscle. 



Sense organs. — All of the soft parts are sensitive to touch, 

 but this sense is especially well developed on the edges of the 

 mantle and of the siphons. There are no definite eyes, but 

 there are pigmented areas in the siphonal region, w^hich are 

 sensitive to light, but which can distinguish no color or form. 

 The osphradium is a small organ at the point of attachment of 

 the gills whose function is supposed, from its situation, to be to 

 test the water as it flows into the gills. It has been suggested 

 that it may possess the undifferentiated chemical sense of smell 

 and taste combined. The otocysts are spherical cavities filled 

 with liquid in which is suspended a solid particle, the otolith. 

 These are balancing organs, serving to keep the clam upright. 

 They are supposed likewise to have an auditory function since 

 pelecypods are able to sense sounds transmitted through the 

 water. 



The following are some of the principal muscles of the body 

 (Figs. 88, 89) : 



I . Mantle muscle. This extends around the edge of the man- 



